Abstract: The pure compound, 6-(-3-methylbut-2-enyl)-6´-7-dihydroxycoumestan (1) isolated from the chloroform extract of the seeds of Psoralea corylifolia Linn. was evaluated for the pesticidal activity against both adults and different instars of Tribolium casteneum Hebrst. under laboratory conditions. The LD₅₀ values for the compound were 910.34, 620.47, 388.45 and 1159.87, 714.88, 404.26 and 1395.70, 740.75, 493.97 and 1605.34, 835.61, 565.83 and 1652.84, 916.79, 729.50 and 1764.32, 994.16, 784.09 and 1678.52, 992.04, 795.67 and 2350.41, 1395.70, 985.12 ppm for the 1st, 2nd, 3rd, 4th, 5th, 6th, adult male and female, respectively at 24, 48 and 72 h post exposure. These results demonstrated that the earlier instars were more sensitive to the compound than those of late instars those follow to those of individual adults.